Shear rate effects on platelet NO elevation. A blood cell suspension, prepared as described in the legend to Figure 2, was perfused over immobilized fibrillar collagen type I at different shear rates for 3 minutes. (A) The number of platelets exhibiting an increase in fluorescence after the perfusion of an identical volume of blood (1.41 mL) was enumerated at the indicated shear rate conditions (250, 400, 1500, 3000 s⁻¹) (*P < .01 vs 250 s⁻¹). (B) The MFI, an index of NO production in DAF-FM–loaded platelets, was quantified for 30 seconds at different shear rates, as indicated (**P < .001 vs 250 s⁻¹). (C) Surface-interacting platelets at 250 and 3000 s⁻¹ were monitored in real time for the first 30 seconds after fluorescence increased. Traces, obtained with MATLAB through computational analysis, are representative of fluorescence intensity of 3 single platelets loaded with DAF-FM. (D) FLUO 3-AM–loaded platelets, prepared as described under Methods, were perfused over immobilized fibrillar collagen type I at different shear rates for 3 minutes. Surface-interacting platelets at 250 and 3000 s⁻¹ were monitored in real time for the first 30 seconds. Traces are representative of fluorescence intensity of 3 single platelets loaded with FLUO 3-AM. The time scale indicates only the temporal trend of the fluorescent signals and not its evolution in relation to platelet-collagen interaction.